Method of producing steel products with improved properties



Feb. 15, 1966 R. A. GRANGE ETAL 3,235,413

METHOD OF PRODUCING STEEL PRODUCTS WITH IMPROVED PROPERTIES Filed NOV. 20, 1961 4 Sheets-Sheet 1 dd 0 1 m m e R e u m 4! e rdp 0T M 0 Man an 49 F 0 0 0 0 0 O 0 0 4 2 8 4 6 3 m n 2 m TEMPERl/VG TEMPERATURE "F (2 Hours) .030, an. 9 M0 60 80 96 COLD REDUG'T/ON Attorney Feb. 15, 1966 R. A. GRANGE ET AL 3,235,413

METHOD OF PRODUCING STEEL PRODUCTS WITH IMPROVED PROPERTIES 4 Sheets-Sheet 2 Filed Nov. 20, 1961 800 1200 TEMPER TEMP. "F (2 Hrs.)

4 Sheets-Sheet 5 R. A. GRANGE ET AL METHOD OF PRODUCING STEEL PRODUCTS WITH IMPROVED PROPERTIES Filed Nov. 20, 1961 400 800 I200 TEMPER TE'MR "F (2 Hrs.)

0 400 800 I200 TEMPER TEMP "F (2 Hrs) INVENTORS. OUENCHED T0 MARTENS/TE, 001.0 RAYMOND A. GRANGE and ROLLED 90% TEMPE/750 JAMES B. MITCHELL CONVENTIONAL OUENCH AND TEMPE'R M QM Affarney Feb. 15, 1966 R. A. GRANGE ETAL 3,235,413

METHOD OF PRODUCING STEEL PRODUCTS WITH IMPROVED PROPERTIES lled Nov. 20, 1961 4 Sheets-Sheet 4 P15 B- ETC-1; EL

0 400 800 I200 0 400 800 I200 TEMPEI? TEMP. "F (2 Hrs.) TEMPE/i TEMP. "E (2 Hrs.)

q 0 l 1 1 /50- -l .l l

0 400 800 /200 0 400 800 /200 TEMPER TEMP. "F (2 Hrs.) TEMPER TEMP. "F (2 Hrs.)

MARTENS/T/C, COLD ROLLED 90% INVENTORS. MARrEA/s/r/qcow ROLLED 75% YMO/VD GRANGE JAMES B. MITCHELL ,amww

Attorney CONVENTIONAL OUENCH AND TEMPE/i United States Patent Olfice 3,235,413 Patented Feb. 15, 1966 This invention relates to the production of steel products having improved microstructure and mechanical properties and more particularly to the production of high strength steel sheet and strip.

Heretofore two methods of improving mechanical properties of steel having been generally used. One is to heat-treat a hardenable steel by heating to austenitize it, quenching from such temperature at a rate sufliciently fast to obtain a martensitic structure and then tempering to achieve the desired combination of strength and ductility. The other method involves cold-working relatively soft material an amount sufiicient to work harden it to the desired degree. Heretofore these two processes have been considered incompatible or mutually exclusive because of the hardened material resulting from the first treatment being too hard and brittle to be cold worked in accordance with the second treatment. We have discovered, however, that under certain critical conditions the two methods can be combined with beneficial effects accruing therefrom.

It is accordingly an object of this invention to produce steel products having enhanced physical properties.

The foregoing and further objects will be apparent from the following specification when read in conjunction with the attached drawings, wherein:

FIGURE 1 is a graph depicting effecting of tempering temperature on yield strength of SAE 4310 steel treated in accordance with the invention as compared to conventional quenching and tempering;

FIGURE 2 is a series of micrographs depicting the microstructure of steel processed in accordance with this invention and tempered for two hours as indicated along with micrographs of the same steel processed in the conventional manner;

FIGURE 3 is a graph depicting the effect of increasing amounts of cold work on steels of this invention;

FIGURES 4 to 7 are graphs individually depicting the effect of tempering temperature on the four steels indicated thereon after treatment in accordance with this invention and after conventional quenching and tempering; and

FIGURES 8 to 11 are graphs depicting the Wide range of mechanical properties available in 4310 steel which has been treated in accordance with this invention. Data for the same steel in the conventionally quenched and tempered condition are included for comparison.

We have discovered that, contrary to accepted beliefs, steel sheet material which has been transformed to a martensitic structure can be cold worked and thereafter tempered to obtain enhanced results provided the steel contains less than .2% carbon. To offset the effect of low carbon on hardenability, the steel should have sufiicient alloy content to cause substantially complete transformation to martensite on quenching. If the alloy content is satisfactorily adjusted to provide sufficient hardenability, the carbon content may be as low as .01%. Along therewith it is essential that the material be drastically cold reduced which, for purposes of the present invention, is understood to mean in excess of 50%. Increasingly higher strengths are obtained with reductions up to at least about 95%. Modification of strength to a lower level with increased ductility is possible by suitable tempering. At temperatures up to 1100 F. the product of this invention has higher strengths than conventionally quenched and tempered material, and material tempered in the neighborhood of 1200 F. to substantially the same strength level as conventionally quenched and tempered material has materially better ductility than conventionally produced product. This is shown by the following Table 1, giving comparative data for SAE 4310. This grade of steel has the following analysis:

It is not necessary, however, in all cases to temper the cold-reduced product as this depends on the amount of ductility required. In contrast to conventional heat-treating methods wherein the sheet product is considerably warped or distorted, the process of this invention provides a sheet or strip which is flat, has a good surface and possesses strengths at least equal to and generally better than that of conventionally produced sheet product.

It may be observed from FIGURE 2 that the microstructure of the steel treated in accordance with this invention is markedly different from the steel as conventionally treated at all levels of tempering. It is this marked difference in microstructure which is responsible for the enhanced mechanical properties of steels treated in accordance with this invention.

From the graphs of FIGURES 4 to 7, it will be noted that for tempering temperatures below about 1100 F., the hardness attainable in the steel treated in accordance with this invention are significantly higher than those attainable by conventional methods. Hardness values follow the same trend as strength.

From FIGURES 8 to 11, it may be noted that a specific combination of strength and ductility can be attained by either varying the amount of cold reduction or varying the tempering temperature or suitable combination of both. Also is should be noted that the loss in ductility in obtaining high hardness values is not great. It is also important to note that on tempering at 1200 F., steels treated in accordance with this invention have extraordinary ductility. Such a Wide range of mechanical properties in a single steel has not heretofore been possible by conventional heat treating methods.

Thus it is seen that our discovery provides sheet or plate material having superior strength or ductility, provided the following requirements are met:

(1) Heat-treating to produce substantially complete martensitic structure of steel containing less than .2% carbon.

(2) Drastically cold reducing such material over 50% and up to (3) Tempering to the desired level of ductility.

Product so produced is characterized by (1) Flatness and good surface.

(2) Very high strength in the as-rolled condition.

(3) Good machinability.

(4) Resistance to softening on tempering at temperatures up to 600 F.

This characteristic enables a product treated in accordance with this invention to be used without loss of strength at service temperatures up to 600 F.

(5) Superior ductility after tempering at temperatures in the neighborhood of 1200 F.

While 'we have shown and described several specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A method of producing high strength steel sheets containing less than .2% carbon and sufficient alloy content to permit quenching to a substantially martensitic structure comprising rolling said steel to plates of the desired intermediate gauges, austenitizing said plates and quenching them to produce a substantially martensitic structure therein, and then drastically cold reducing them in excess of 50% to sheet gauge to flatten them and produce an increase in hardness therein.

2. The method of claim 1 including the step of tempering said sheets to the desired strength and ductility level after said drastic cold reducing.

3. A method of producing high strength steel sheets containing less than .2% carbon and sufficient alloy content to permit quenching to a substantially martensitic structure characterized by excellent ductility comprising rolling said steel to the desired intermediate gauge,

austenitizing and quenching said steel to produce a substantially martensitic structure therein, cold reducing the steel having said martensitic structure in excess of to increase the hardness thereof and then tempering said cold reduced steel at about 1200 F. to substantially improve the ductility thereof.

4. A steel sheet containing less than .2% carbon, the steel of said sheet having a martensitic microstructure, said sheet having been improved by cold reducing at least 50% following the substantially complete transformation of said sheet to martensite.

5. A steel sheet possessing superior ductility containing less than .2% carbon, the steel of said sheet having a martensitic microstru-cture, said sheet having been improved by cold reducing at least 50% and tempering following the substantially complete transformation of said sheet to martensite. I

References Cited by the Examiner UNITED STATES PATENTS 2,717,846 9/1955 Harvey 14812.4 2,924,544 2/1960 Nachtman 14812 2,943,463 4/1960 Schmatz et al. 14812.4 3,053,703 9/1962 Breyer 148--12.4

7 OTHER REFERENCES The Iron Age, by Harvey, Dec. 27, 1951 (pp. 71 relied upon).

DAVTD L. RECK, Primary Examiner.

RAY K. WINDHAM, Examiner. 

1. A METHOD OF PRODUCING HIGH STRENGTH STEEL SHEETS CONTAINING LESS THAN .2% CARBON AND SUFFICIENT ALLOY CONTENT TO PERMIT QUENCHING TO A SUBSTANTIALLY MARTENSITIC STRUCTURE COMPRISING ROLLING SAID STEEL TO PLATES OF THE DESIRED INTERMEDIATE GAUGES, AUSTENITIZING SAID PLATES AND QUENCHING THEM TO PRODUCE A SUBSTANTIALLY MARTENSITIC STRUCTURE THEREIN, AND THEN DRASTICALLY COLD REDUCING THEM IN EXCESS OF 50% TO SHEET GAUGE TO FLATTEN THEM AND PRODUCE AN INCREASE IN HARDNESS THEREIN.
 2. THE METHOD OF CLAIM 1 INCLUDING THE STEP OF TEMPERING SAID SHEETS TO THE DESIRED STRENGTH AND DUCTILITY LEVEL AFTER SAID DRASTIC COLD REDUCING. 